* {
    margin: 0;
    padding: 0;
}

.bigbox {
    position: relative;
    width: 800px;
    height: 400px;
    border: 1px solid red;
}

.smallbox {
    width: 100px;
    height: 100px;
    left: 0px;
    top: 0px;
    background: skyblue;
}